1. Field of the Invention
The present invention relates to an axle driving device including a hydrostatic transmission (hereinafter, referred to as an “HST”) having a hydraulic pump and a hydraulic motor which are in fluid communication with one another through a closed circuit within a center section of the HST, an axle capable of being driven by driving power of various speeds from the HST, and a common housing which houses the HST and the axle. More particularly, the present invention relates to a structure for mounting a magnet capable of removing iron powder from lubrication oil to be supplied as a working fluid from an oil reservoir within the housing to the closed circuit.
2. Related Art
Conventionally, an axle driving device includes an HST housed within a housing which is filled with lubrication oil which also serves as a working fluid. The lubrication oil may contain iron powder which has been cut away from transmission gears provided between the HST and the axle during the rotation thereof. The lubrication oil may also contain iron powder in the form of work dusts which may have been originally adhered to the members constituting the axle driving device. This iron powder may flow within the housing together with the lubrication oil because of changes in the volume of the lubrication oil caused by the oil-temperature change, starting and stopping of the HST, or agitation caused by the operation of the HST and the rotations of the transmission gears, among other things. As a result, the iron powder may be introduced into the closed circuit within the HST together with the lubrication oil as the working fluid, which may reduce the durability of the HST by, for example, causing damage to its sliding surface.
Known techniques for efficiently eliminating iron powder include attracting the iron powder to a magnet mounted within a housing and filtering iron powder with a common oil filter made of a porous material. Japanese App. No. 2000-108699 discloses an exemplary axle driving device which employs such an iron-powder elimination technique using a magnet. The axle driving device taught by App. No. 200-108699 is provided with plate-shaped magnets which are placed on respective wall portions of the housing and also near an oil filter connected to an oil hole leading to a closed circuit in a center section.
In such an assembly, the magnets near the oil filter are mounted on upper and lower cover plates for sandwiching the oil filter at its upper and lower portions. For example, in the case of the lower cover plate, the magnet is attracted and secured through its magnetic force to an upper surface or a lower surface of the lower cover plate made of a magnetic material such as an iron plate or a nickel plate, while, in the case of the upper cover plate, the magnet is hung by providing a mounting member such as a bridge or a pedestal.
Requiring additional components to securely mount the magnets, such as upper and lower cover plates, increases component costs and unnecessarily complicates assembly. Further, where a plate-shaped magnet is mounted to the lower cover plate by attraction thereto, only one surface is free as an iron-powder attracting surface, thereby presenting a smaller absorption area and poor iron-powder reduction efficiency. In addition, in this configuration, the magnet may not be fixed to the cover plate firmly. Accordingly, in the event that sudden vibrations act on the cover plates, the magnet may freely move and impinge on peripheral components such as the oil filter, thus inducing fractures or other damage to these components. Accordingly, a need exists for a structure capable of overcoming one or more of the limitations described above.